Hi Gabriele,
There are a few methods to do this. Actually, there are a bunch, here are a few.
One, you could clean up the crack with a chisel and fill it with epoxy. You can mix colors into the epoxy if desired, you can also fill the crack up nearly to the top *after* sufacing the topside to nearly final deminsion and then, after the first epoxy has dried, use the sanding dust from the wood itself to mix into fresh epoxy and fill to the surface.
Second, you can use a chisel to cut the space needed to inlay a fresh piece of the wood. Try for a shape that you would not have too much trouble in cutting a new piece of wood to fill. In other words, as stright of a "dado" as possible, even if the crack is not straight the dado housing can be. This should be epoxied in for strength. The dado should be drawn out on the surface and you would need to try to cut as flat a bottom as possible.
Work your way out to the line, but stay a little from it. Once the dado is nearly even, then you can cut to the line in your final slices. Try for cuts at this point that are straight down at the line. To make the flat bottom you would turn the chisel so the bevel is down to flaten the bottom. IF you had a #71, this is when you would use it, to clean up the bottom to the same depth, but it isn't a necessary tool for this repair.
Then you would cut a piece to inlay slightly larger and using a plane or chisel or sandpaper slowly make it fit the dado. Pay attention to the grain and try to match the repair as good as possible.
Third, which could be heresy here in the handtool forum, would be to use an electric router with guides clamped to the top to make sure the router doesn't cut where you don't want it to. You would need to make several passes with the router, lowering the bit a little after each pass.
All three of the methods should be done after most leveling of the top is done.
